Field of the Invention
The present disclosure relates to semiconductor substrates, semiconductor devices, and manufacturing methods thereof. More particularly, the present disclosure relates to semiconductor substrates having a GaN layer formed on a substrate, semiconductor devices, and manufacturing methods thereof
Discussion of the Background
A light emitting diode (LED) that has a gallium nitride (GaN) based semiconductor may be used for various applications, such as signal devices, backlight units for liquid crystal panels, and the like. It is known that light emitting efficiency of LEDs is affected by dislocation density and defects in a crystal. Although GaN-based semiconductor crystals may be grown on a heterogeneous substrate, such as sapphire or the like, lattice mismatch and differences in thermal expansion between the GaN layer and the substrate may occur, causing a high dislocation density or an increase in defect density.
The crystal growth of a GaN-based semiconductor may be carried out on a homogeneous substrate, such as a GaN substrate and the like. However, a high dissociation rate of nitrogen in GaN may obstruct formation of a GaN melt, thereby making it difficult to form a GaN substrate. Although mechanical polishing, laser delamination or the like may be used to separate the GaN substrate from a GaN bulk crystal grown for the GaN substrate, it may be difficult to produce a GaN substrate having a practical size. Particularly, the laser delamination may require a significantly long period of time to perform and cause an increase in the cost of the GaN substrate.
GaN crystal growth is shown and described in “Polycrystalline GaN for light emitter and field electron emitter applications,” by S. Hasegawa, S. Nishida, T. Yamashita, H. Asahi, (Thin Solid Films 487 (2005), pp 260-267) (hereinafter “Hasegawa, et al.”), and “Buried Tungsten Metal Structure Fabricated by Epitaxial-Lateral-Overgrown GaN via Low-Pressure Metalorganic Vapor Phase Epitaxy,” M. Haino, et al., (Jpn. J. Appl. Phys., 39 (2000) L449) (hereinafter “Haino, et. al.”), which are hereby incorporated by reference in their entirety for all purposes as if fully set forth herein. For example, GaN crystals are respectively grown on quartz substrates, high-melting-point metal substrates of Tungsten (W), Molybdenum (Mo), Tantalum (Ta), and Niobium (Nb), and Silicon (Si) substrates using plasma assisted molecular beam epitaxy.
Since it may be difficult and costly to fabricate the GaN substrate, semiconductor devices such as LEDs or laser diodes are generally manufactured by growing a GaN layer on a heterogeneous substrate, such as sapphire and the like. However, as mentioned above, the high dislocation density or the increase in defect density may degrade the light emitting efficiency of the LED. In addition, the sapphire substrate has a lower thermal conductivity than the GaN substrate, and may cause deterioration in heat dissipation properties of a device. Thus, the use of a sapphire substrate for LEDs or laser diodes may limit the operational lifespan thereof.